Semi-chemical pulp is produced generally by the so-called cold soda process wherein wood chips are impregnated with sodium hydroxide and after storage for some time in a reaction vessel, they are defibrated, which usually is effected in a disc-equipped grinding apparatus, whereupon the ground pulp is relieved from residues of chemicals by washing.
Sometimes in the production of mechanical pulp, the wood is impregnated with a weak alkali solution in order to swell the wood fibres and to increase the strength of the fibre coherence in the final product. However, all alkali treatment results in a deterioration of the optical properties of the pulp such as brightness and light-scattering properties thereof. These changes are particularly conspicuous when deciduous wood is used for making pulp in this manner.